[Explicit Module Builds] Support Versioned PCMs

There exists a strict limitation that a compatible PCM must have the exact same target version with the loading module (either a Swift module or another PCM). This means that each PCM may have to be built multiple times, once for each unique (SDK, architecture, toolchain) tuple of some loading module.

This PR introduces support for generating versioned PCMs for each loading module.
Because now there is more state that we needed to keep track of when generating jobs for module dependencies, this state, along with the Inter-Module Dependency Graph, now live in the `ExplicitModuleBuildHandler`, which is constructed on-demand by the driver in the explicit module build mode. The handler is responsible for generating dependency build jobs and for vending out dependency modules as inputs/command-line options whenever the driver creates a compile job in Explicit Module Build Mode.

Because this PR rewrites/refactores the majority of existing Explicit Module Build code, I tooke the opportunity to re-organize some of the files as well.

Resolves rdar://problem/63944640

Author: artemcm

Sources/SwiftDriver/CMakeLists.txt

@@ -7,8 +7,10 @@
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 add_library(SwiftDriver
-  "Dependency Scanning/ModuleDependencyBuildGeneration.swift"
-  "Dependency Scanning/InterModuleDependencyGraph.swift"
+  "Explicit Module Builds/ClandModuleBuildJobCache.swift"
+  "Explicit Module Builds/ExplicitModuleBuildHandler.swift"   
+  "Explicit Module Builds/InterModuleDependencyGraph.swift"
+  "Explicit Module Builds/ModuleDependencyScanning.swift"
 
   Driver/CompilerMode.swift
   Driver/DebugInfo.swift
@@ -17,7 +19,6 @@ add_library(SwiftDriver
   Driver/ModuleOutputInfo.swift
   Driver/OutputFileMap.swift
   Driver/ToolExecutionDelegate.swift
-  Driver/ModuleDependencyScanning.swift
 
   Execution/JobExecutor.swift
   Execution/ParsableOutput.swift

Sources/SwiftDriver/Dependency Scanning/ModuleDependencyBuildGeneration.swift

@@ -26,6 +26,7 @@ public struct Driver {
     case conflictingOptions(Option, Option)
     // Explicit Module Build Failures
     case malformedModuleDependency(String, String)
+    case missingPCMArguments(String)
     case missingModuleDependency(String)
     case dependencyScanningFailure(Int, String)
 
@@ -49,6 +50,8 @@ public struct Driver {
       // Explicit Module Build Failures
       case .malformedModuleDependency(let moduleName, let errorDescription):
         return "Malformed Module Dependency: \(moduleName), \(errorDescription)"
+      case .missingPCMArguments(let moduleName):
+        return "Missing extraPcmArgs to build Clang module: \(moduleName)"
       case .missingModuleDependency(let moduleName):
         return "Missing Module Dependency Info: \(moduleName)"
       case .dependencyScanningFailure(let code, let error):
@@ -169,10 +172,10 @@ public struct Driver {
   /// This will force the driver to first emit the module and then run compile jobs.
   public var forceEmitModuleInSingleInvocation: Bool = false
 
-  /// The module dependency graph, which is populated during the planning phase
-  /// only when all modules will be prebuilt and treated as explicit by the
-  /// various compilation jobs.
-  public var interModuleDependencyGraph: InterModuleDependencyGraph? = nil
+  /// Handler for constructing module build jobs using Explicit Module Builds.
+  /// Constructed during the planning phase only when all modules will be prebuilt and treated
+  /// as explicit by the various compilation jobs.
+  public var explicitModuleBuildHandler: ExplicitModuleBuildHandler? = nil
 
   /// Handler for emitting diagnostics to stderr.
   public static let stderrDiagnosticsHandler: DiagnosticsEngine.DiagnosticsHandler = { diagnostic in

Sources/SwiftDriver/Driver/Driver.swift

@@ -0,0 +1,38 @@
+//===----------------- ClangModuleBuildJobCache.swift ---------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// Maps tuples (ModuleDependencyId, [String]), where [String] is an array of pcm build opitions,
+/// to the job used to build this module with these specified build options.
+struct ClangModuleBuildJobCache {
+
+  /// As tuples cannot be Hashable, wrap (ModuleDependencyId, [String]) into a struct pair
+  private struct ModuleArgumentPair : Hashable {
+    let moduleId: ModuleDependencyId
+    let buildArgs: [String]
+  }
+
+  private var cache: [ModuleArgumentPair: Job] = [:]
+
+  var allJobs: [Job] {
+    return Array(cache.values)
+  }
+
+  subscript(index: (ModuleDependencyId, [String])) -> Job? {
+      get {
+        return cache[ModuleArgumentPair(moduleId: index.0, buildArgs: index.1)]
+      }
+      set(newValue) {
+        cache[ModuleArgumentPair(moduleId: index.0, buildArgs: index.1)] = newValue
+      }
+  }
+}
+